Shallow trench isolation structures are used in semiconductor device fabrication to isolate active areas of the semiconductor substrate. Typically, shallow trench isolation structures are formed by etching the top surface of the semiconductor substrate so as to form trenches that extend into the semiconductor substrate. These trenches surround areas of the semiconductor substrate that are commonly referred to as “active areas.” A layer of oxide is then deposited to fill up the trenches, electrically isolating active areas from each other.
Conventional shallow trench isolation structures usually include trenches that have a flat horizontal bottom and that have vertical sidewalls. The sharply angled bottom corners of conventional shallow trench isolation structures result in thermal-mechanical stress buildup concentrated at the bottom corners. This corner stress buildup can result in localized stress-related defects. In severe cases, stresses from subsequent process steps and from thermal cycling can cause dislocation and cracking. This can cause device failure, resulting in reduced manufacturing yield and increased manufacturing costs.
Another problem that can arise with respect to conventional shallow trench isolation structures is electrical field buildup enhancement at top corners of the active area. The sharper the corner is, the greater enhancement of the field buildup will be. As a result, the corner portion of the transistor will have lower threshold voltage than the the main channel portion of the transistor. This phenomenon, commonly referred as “kink effect,” can produce early turn-on of transistors and undesired cross-talk between adjacent devices.
One method for reducing stress buildup and kink effect is to form an oxide liner by performing an oxidation step prior to filling the trench. However, conventional processes for forming oxide trench liners use high temperature processes (often at temperatures of greater than 1,100 degrees centigrade), which stress the substrate, resulting in undesired stress buildup.
After the formation of conventional shallow trench isolation structures, implantation steps are performed so as to form various doped regions. During these subsequent implant steps, resist masks are exposed and developed to define the areas that are to be doped. However, when the resist thickness is not sufficient to block incoming light, some of the light passes through the resist and into the trenches. This light reflects off of the sidewalls of the trench so as to cause resist notching and lifting. Resist notching exposes areas of the active areas that are supposed to be covered by resist, causing undesired implantation. Lifting results in resist covering areas that were supposed to be exposed, preventing the implantation of desired impurities.
One method for preventing this problem is to increase the resist thickness during implant process steps. However, this can result in bridging and may not be practical when features less than 0.13 micron must be formed.
Accordingly there is a need for a shallow trench isolation structure and a method for forming a shallow trench isolation structure that reduces localized stress-related defects and “kink effect.” Also, there is a need for a shallow trench isolation structure and a method for forming a shallow trench isolation structure that will reduce or eliminate notching and lifting in subsequently formed resist masks. The present invention meets the above needs.